1. Field of the Invention
The present invention relates to the separation of solutes dissolved in liquids and gases of differing masses and to the separation of particulates suspended in gases or liquids and to the separation of liquids of different densities.
2. Discussion of the Prior Art
Diffusion has long been employed to separate molecules. Graham ["On the law of diffusion of gases," Philosophical Magazine, Vol. 2, pp. 175-351 (1833)], who first related the molecular diffusion coefficient to the square root of molecular weight, separated gases based upon this principle in the 19th century. Hertz developed a technique based on diffusion to separate gases in a countercurrent system [Z. Physik., Vol. 19, p. 35 (1923) and Vol. 91, p. 810 (1934)].
This technique was extended to liquids by Lange [Z. Naturwiss., Vol. 16. p. 115 (1928) and Vol. 17, p. 228 (1928)]. There is evidence that students of Lange expanded on this work and reported their results in papers published in East Germany that are not readily available. East German Patent No. 54,339 describes the method of Lange as one involving the enhancement of the diffusion rate by oscillations and regeneration of the solvent by successive distillation and condensation.
Dreyer et al [Die Steigerung des Diffusions-transportes durch Pulsationsdiffusion, Z. Naturforsch., Vol. 23, pp. 498-503 (1968) and Die Bestimmung von Diffusionskoeffizienten nach der Pulsationsmethode, Z. Naturforsch., Vol. 24, pp. 883-886 (1969)] describe a system for determining the diffusion coefficients of solutes such as KC, NaCl and CaCl.sub.2 comprising two containers connected by a capillary and a mechanism for creating pulsating oscillations in the liquid contained in the capillary. The authors discovered an enhancement of transport of several orders of magnitude.
Modified principles of diffusion are used industrially today, especially to separate isotopes of uranium. Diffusion has been used to separate solutes in liquid solution; however, the efficacy of the process is low because the molecular diffusion coefficient of solutes in liquids is about five orders of magnitude smaller than the diffusion coefficient of gases in a gaseous phase, thus reducing the possible yield for a given configuration.
Enhanced diffusion (or dispersion) by oscillatory motion of a fluid finds its roots in the theoretical work by Watson [J. Fluid Mech., Vol. 133, p. 233 (1983)] who himself expanded on a study by Taylor on the dispersion of solutes in steady laminar flow [Proc. R. Soc. London Ser. A 219, p. 186 (1953)]. Kurzweg et al recently described the conditions of optimal transport in gases and liquids by proper tuning of the experimental variables [Phys. Fluids, Vol. 29, p. 1324 (1986)]. See also Harris et al, Chem. Eng. Sci., Vol. 22, pp. 1571-1576 (1967); and Kurzweg et al, Phys. Fluids, Vol. 27, pp. 1046-1048 (1984). The principle evolved from the work of Taylor et al (Proc. R. Soc. London Ser. A 219, pp. 186-203 (1953). The concept has found medical application [Slutzky et al, Science, Vol. 209, pp. 609-611 (1980) and Chang, J. Appl. Physiol., Vol. 56, pp. 556-563 (1984)].
The general principle involved may be described thusly: The oscillation of a fluid column in a tube generates a large surface between the oscillating core and the boundary layer which is essentially not moving. This surface is made available for diffusion. The theory predicts that, under certain conditions, the dispersion coefficient (i.e., the effective diffusion coefficient) is proportional to:
(a) the square of the frequency (in gases) or to the square root of the frequency (in liquids) or, under some conditions, to a power function of frequency between 0.5 and 2.0; PA1 (b) the square of the oscillation amplitude .DELTA.x (defined hereinbelow); or PA1 (c) either the molecular diffusion coefficient D.sub.m (in liquids) or the reciprocal value of this coefficient 1/D.sub.m (in gases) or, under some conditions, a value between these extremes.
Systems have been developed for the industrial application of these techniques for separating gases and solutes. See Kurzweg and Jaeger, U.S. Pat. No. 4,770,675; Jaeger, U.S. Pat. No. 4,844,814; Jaeger and Kurzweg, "Determination of the longitudinal dispersion coefficient in flows subjected to high frequency oscillations," Phys. Fluids, Vol. 26, pp. 1380-1382 (1983); and Kurzweg and Jaeger, "Tuning effect in enhanced gas dispersion under oscillatory conditions, " Phys. Fluids, Vol. 29, pp. 1324-1326 (1986).
As discussed above, diffusion against a steady flow was described by Hertz in 1923 with the intention of separating gases of differing molecular weights. The solution of the Fick equation for a counterflow with a flow rate Q.sub.c gives EQU ln (c.sub.1 /c.sub.2)=LQ.sub.c /AD.sub.m ( 1)
where c refers to the concentration, L to the length of the diffusing path in cm, A to the cross-sectional area in cm.sup.2 and D.sub.m to the molecular diffusion coefficient in cm.sup.2 /sec; the indices 1 and 2 refer to the high and low ends of the diffusion gradient, respectively. For comparison, the classical equation describing diffusion without counterflow is EQU c.sub.1 -c.sub.2 =qL/AD.sub.m ( 2)
where q is the flux of the diffusing molecule(s). Equation (1) used by Hertz provides potentially superior separation because of the logarithmic relationship; however, theoretically, the method provides for no flux, i.e., for no technically usable yield.
It is an object of the present invention to provide a hybrid system which combines the principles embodied in Equations (1) and (2) to provide an improved method for separating gases and solutes of differing masses.